Local vascular pathway for progesterone transfer to the brain after nasal administration in gilts

In the present study we examined whether local transfer of intranasally administrated tritiated progesterone (3H-P4) would increase its concentration in blood supplying the brain and hypophysis in comparison with other organs. Additionally, the effect of estrous cycle on the P4 transfer was evaluated on isolated gilts' heads. In the first experiment 3H-P4 was instilled into the nasal cavities of anaesthetized, immature pigs (n=10). Simultaneous blood samples were collected for radioactivity measurement every minute from the same occluded carotid artery through two catheters; one catheter was pointed towards the head, the other one towards the heart. In eight animals the ratio calculated between the 'head' and 'heart' samples was significantly (p<0.05) higher than 1 and reached a mean (+/- SEM) level of 3.23 +/- 0.81. In two animals a much higher ratio was observed. A head/heart ratio>1 indicates an existence of local transfer of 3H-P4 from venous blood to the carotid blood. In the second experiment, heads of 26 mature, cycling gilts were perfused through the right carotid artery with autologous blood. The outflow from the left carotid artery was collected as 1 min samples. 3H-P4 was infused into the angularis oculi veins. Transfer of 3H-P4 from the venous blood into the arterial blood reached the mean (+/- SEM) level of 4.11 +/- 1.08 pg/ml on days 2-4, 3.2 +/- 0.70 on days 17-21 and 0.94 +/- 0.22 pg/ml on days 15-16 of the estrous cycle. No 3H-P4 transfer was observed on days 9-11. These findings demonstrate that nasally administered progesterone can reach the brain in locally higher concentration through the vascular pathway. Moreover, the between-vessel transfer of P4 is significantly affected by the stage of the estrous cycle.